222 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



older groups. The April 11 and 13, 1932, samples 

 seem to bear out this expectation as there were -no 

 females in age group II but they accounted for 14.3 

 and 39.4 percent, respectively, of all fish in age 

 groups III and IV. Although other factors no 

 doubt affected the sex ratio in the April 1932 

 samples (see section on sex ratio, p. 260), segrega- 

 tion on the basis of maturity must have played an 

 important part. 



Inadequate sampling because of segregation 

 according to maturity is of little unportance in the 

 present study since only one collection employed 

 in the study of age and growth was taken from the 

 spawning rim (1932 collection). In this material 

 the reliability of the data for the group-Ill 

 females only is open to question. 



Since maturity and length are closely related, it 

 is possible that segregation according to maturity 

 may be a source of destruction in the spawning- 

 run fishery of fish with more rapid growth. The 

 effects of this higher mortality of fish with rapid 

 growth on comparisons of the growth histories of 

 fish of different ages are similar to the effects of 

 the selective destruction of rapidly growing fish 

 associated with gear selectivity (p. 221). 



SELECTIVE DESTRUCTION ACCORDING TO THE 

 LEGAL SIZE LIMIT 



The imposition of a minimum legal size limit 

 does much to reduce the effect of selection by gear 

 through the protection of the faster-growing but 

 still illegal-sized individuals, but at the same time 

 adherence to a legal size limit produces a similar 

 selective effect of its own. As the fish reach the 

 minimum legal size limit they are subject to re- 

 moval by the commercial fishery. Consequently, 

 the faster-growing individuals are exposed to this 



source of destruction earlier in life than are those 

 of slower gi'owth. In a heavily exploited fishery, 

 successive samples of a year class, then, may be 

 composed of fish with successively slower growth 

 as a consequence of continued sorting according 

 to size. 



The manner and extent to which the selective 

 destruction of yellow perch accordmg to legal size 

 Umit may give rise to discrepancies between the 

 calculated growth histories of different age groups 

 are brought out by the data of table 6. Effects of 

 the elimination of different percentages of legal- 

 sized fish ° on the determination of the growth 

 histories of three age groups also are shown. 

 From the data of table 6 it is obvious that the con- 

 tinued removal of legal-sized yellow perch in the 

 commercial fishery will bring about a decrease in 

 the calculated growth rates of an age group. 

 The first-year computed lengths were affected the 

 least. The exclusion of all legal-sized fish reduced 

 the fh-st-year length by only 0.1 inch in the 1928 

 group II and 0.2 inch in the 1929 group II but 

 brought about a 0.2-inch increase in the first-year 

 length of the 1928 group III. On the other hand, 

 the effect of the elimination of legal-sized yellow 

 perch on the determination of the calculated 

 lengths at the end of the second and third years of 

 Ufe was pronoimced. The decreases in the second- 

 year length with aU legal-sized fish excluded were 

 as high as 0.7 inch (1929 group II); the decreases 

 in the thu'd-year length were as high as 0.9 inch 

 (1928 group III). When lesser percentages of 



" A size limit of 8M inches was employed in the separation of legal and 

 undersized fish in all three age groups although a 9-inch limit was actually 

 in effect in 1928. Since most of the 1928 samples were taken in the summer 

 before completion of the season's growth and most of the 1929 samples were 

 taken in the fall, presumably after completion of the season's growth, it was 

 believed that the data for all age groups would be made more nearly com- 

 parable by the use of a single size limit. 



Table 6. — Effect of excluding legal-sized fish in determining growth histories of Lake Erie yellow perch 



[Legal size: BH inches] 



1 Length at capture in fall (see footnote 0, above). 



